hornblende

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hornblende:

see amphiboleamphibole
, any of a group of widely distributed rock-forming minerals, magnesium-iron silicates, often with traces of calcium, aluminum, sodium, titanium, and other elements.
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Hornblende

 

(1) The most common rock-forming mineral of the group of monoclinic amphiboles (common hornblende). Its approximate chemical composition is Ca2Na[Mg, Fe]4[Al,-Fe][(Si, Al)4O11]2[OH, F]2, with substantial variations in the ratios of basic components and with various element impurities (Na <- K, Ba, Rb; Ca ← TR, Y, Sr, U, W, Mo; Al ← Ga; Mg ← Li; Fe ← Zn, Sc). Hornblende crystallizes in the monoclinic system. It forms long, prismatic needles, irregular grains, and fibrous aggregates of green, black, or brown color. It has a hardness of 5.5–6 on Mohs’ scale and a density of 3,100–3,300 kg/m3. Hornblende is characterized by two directions of well-developed cleavages intersecting at an angle of 124°.

Common hornblende is characteristic of intrusive rocks (granodiorites, syenites, diorites, hornblende gabbros), effusive rocks (andesites, basalts, porphyrites), and metamorphic (am-phibolites, hornblende gneisses) rocks. Common hornblende is a relatively stable mineral, but when acted upon by hydrothermal solutions it alters to chlorite, epidote, calcite, and quartz. In the process of weathering, it decomposes to form opal, carbonates, and various other minerals.

(2) The subgroup of calcium aluminiferous amphiboles that includes common hornblende, edenite, oligoclase, pargasite, and hastingsite.

REFERENCES

Deer, W. A., R. A. Howie, and J. Zussmann. Porodoobrazuiushchie mineraly, vol 2. Moscow, 1965. (Translated from English.)
Lodochnikov, V. N. Glavneishie porodoobrazuiushchie mineraly, 5th ed. Moscow, 1974.

A. S. MARFUNIN

hornblende

[′hȯrn‚blend]
(mineralogy)
A general name given to the monoclinic calcium amphiboles that form an extensive solid-solution series between the various metals in the generalized formula (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2.

hornblende

a black or greenish-black mineral of the amphibole group, found in igneous and metamorphic rocks. Composition: calcium magnesium iron sodium aluminium aluminosilicate. General formula: (Ca,Na)2.3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2
References in periodicals archive ?
The complex zonation of amphiboles suggest that the Sona amphibolites had undergone an counterclockwise regional metamorphism progressive from the greenschist facies to the hornblende plagioclase amphibolite facies at low to medium pressure and temperature (350 to 650[degrees]c).
This age is quite similar with that of 62 Ma on hornblende and that of 51 Ma on feldspar provided both by [sup.40]K/[sup.40]Ar dating on another Azuero quartz diorite stock (Kessler et al., 1977).
From combined field, geochemical and isotopical studies it has been concluded that the mantle-derived mafic magmas from which the MMEs crystallized were likely mostly formed by mafic-felsic interaction and mingling, or prior to the mixing crystal fractionation of hornblende ([+ or -] pyroxene) and Fe-Ti oxide.
Hornblende was also dated from the amphibolites in subophiolitic metamorphic rocks that yielded a good K Ar age of 80.5 5.3 (Sawada et al.
Anthophyllite schists are restricted to hornblende schists, and they appear as anastomosed irregular lenses, with thicknesses between 4 and 11 cm, composed of translucent and fibrous minerals parallel to schistosity (Figure 2i).
the mineral assemblages at equilibrium found in greenschists, hornblende schists, garnet amphibolites, and amphibolites, and 2.
The first one corresponds to epidote-amphibolite facies, represented by hornblende + epidote group minerals + quartz + albite [+ or -] garnet (Figure 3c).
The first one corresponds to amphibolite facies (Figure 3e), defined by hornblende + plagioclase [+ or -] garnet [+ or -] quartz.
The mineral assemblage corresponds to anthophyllite + hornblende + quartz + plagioclase + chlorite + epidote minerals, with titanite and calcite as accessory minerals.
This deformation phase is defined by actinolite, chlorite and muscovite in the greenschists (Figures 3a y 3b), by hornblende in the hornblende schists (Figure 3c) and the amphibolites (Figure 3e), by chlorite, muscovite and graphite in quartz-muscovite-chlorite schists or mica schists (Figures 3h and 30, and by hornblende and anthophyllite in the anthophyllite schists (Figure 3j).
Microsopically, this deformation phase also was recorded in the hornblende schists.
Thereby, in the Arquia Group type section, the greenschists change in a transitional form to hornblende schists westward, and the amphibolites appear further to the west.